During many of the steps involved in the manufacture of integrated circuits, features are defined by photolithographic methods. Careful control of the features is often essential for good performance of the integrated circuit. The features are difficult to measure using conventional optical techniques. The features are small, often about 1 micrometer in cross section, and the feature edges are not always perfectly straight and vertical (i.e., perpendicular to the underlying substrate). Often, the feature edges are sloped or irregular along the length of the feature. Consequently, a precise definition of the terms "feature width" and "feature height" is difficult.
Various techniques have been developed for measuring the pitch and width of features on a substrate. These include the use of scanning electron microscopes (both high voltage, using energies of 15-30 kV, and low voltage, using energies of typically less than 2 kV), scanning probe microscopes (Atomic Force Microscopy, etc.), and optical microscopes. Combinations of these techniques are often employed.
In the case of integrated circuits, a section through the circuit, including the feature to be examined, is generally cut and the exposed cross section is subjected to the selected measurement technique or techniques. Alternatively, a portion of the substrate is gouged out of the wafer, carrying with it the topographical feature to be examined. One method of sectioning an integrated circuit includes the use of a Focused Ion Beam (FIB) tool. An FIB produces a finely focussed ion beam, such as a beam of gallium ions, which can be directed at the wafer.
These method of measuring feature geometry, or "metrology" have a number of disadvantages. First, the taking of a section through the entire wafer destroys the wafer on which the circuit is formed. As wafers are manufactured with increasingly large diameters (20 cm, and above) the costs of the wafers increase accordingly and sectioning also becomes more difficult. It is therefore desirable that a non-destructive testing method be developed.
Second, the sectioning process used to cut the section tends to change the cross section of the feature to be examined, especially in the case of relatively soft materials, such as photoresist. FIB is particularly damaging to the soft photoresist material, leading to deformation of the cross section. Thus, the dimensions measured may not correspond to those which were present on the wafer prior to sectioning.
Third, the taking of a section permits only one cross section of the feature to be examined at the end of the section. It does not allow a measurement of the changes in cross section along the length of the feature. Additionally, the section taken may not be representative of the feature as a whole.
The present invention provides a new and improved method for examining a feature which overcomes the above-referenced problems, and others.